Place an oven rack in the bottom position and preheat to 400F. Flip over a baking sheet and line with parchment paper. Unroll the pie dough on a work surface, then stack the two rounds, gently pressing to adhere. Roll out into a 12-inch round and transfer to the prepared baking sheet.
Combine the blackberries, cornstarch salt, 1/4 cup granulated sugar, the lemon zest and 1 tablespoon lemon juice in a medium bowl and gently mix until the cornstarch has dissolved
Combine the cream cheese, sour cream, vanilla, egg yolk, remaining 1/4 cup of granulated sugar, and 1 tablespoon of lemon juice into another medium bowl. Beat with a mixer on medium-high speed until combined and smooth, 2 to 3 minutes
Use a slotted spoon to remove six or seven blackberries into a separate bowl. Spoon the remaining blackberry mixture into the center of the pie dough, leaving a 2-inch border. Drizzle the cream cheese mixture on top, leaving the border uncovered. Gently fold in and pleat the dough edge, taking care not to rip it. Scatter the reserved blackberries on top of the filling. Brush the crust with the beaten egg, then sprinkle with the coarse sugar.
Bake the galette until the crust is a deep golden brown, the cheesecake is set and the blackberries are slightly bubbling through the cheesecake in spots, 30 to 35 minutes. Transfer the baking sheet to a rack and let the galette cool completely about 1 hour.